1. Field of the Invention
This invention relates to a flexible printed circuit that is a functional component used chiefly in electric and electronic equipments and a process for producing the same.
2. Description of the Conventional Art
A flexible printed circuit (FPC) has enjoyed wide use in the field of electric and electronic equipment. A flexible printed circuit generally has a laminate structure comprising a plastic film and a conducting layer, such as a metal foil. An illustrative example of the flexible printed circuit is shown in FIG. 6(A), in which a flexible printed circuit 1 comprises a plastic base film 4 made of polyimide, polyester, etc. having formed thereon a conductor circuit 3 by a printing process, a subtracting process, etc., and the conductor circuit 3 is covered with a plastic cover film 2 similar to the base film 4. Incidentally, the adhesive layers are omitted in FIG. 6(A).
FIG. 6(B) shows a general structure of the flexible printed circuit. In FIG. 6(B), an adhesive layer 6 is provided on a plastic base film 4 via which a conductor circuit 3 is laminated, and another adhesive layer 6 is also provided on the side of the plastic cover film 2. That is, the plastic films 2 and 4 are bonded together with their respective adhesive layers facing each other. Further, part of the plastic cover film 2 can be removed (indicated by numeral 5 in FIGS. 6(A) and 6(B)) if necessary for electrical connection of the conductor circuit 3 to other electronic components.
Flexible printed circuits are required to have dimensional accuracy because electronic components such as semiconductor devices are to be mounted thereon at small pitches or they are to be connected to electronic equipment via connectors, etc. at small pitches. Since a flexible printed circuit has a laminate structure comprising plastic films and a conducting layer as typically shown in FIGS. 6(A) to (C), their dimensional accuracy largely depends on the dimensional stability of the plastic films used. The plastic films for use in the flexible printed circuit are therefore required to have dimensional stability, particularly against heat because they are usually handled under a high temperature condition in the manufacture of flexible printed circuits.
It is necessary to select plastic films having satisfactory dimensional stability before a flexible printed circuit with improved dimensional accuracy can be produced. Close evaluation on the dimensional stability of plastic films would be essential therefor.
However, in the practice of manufacture of flexible printed circuits using conventional plastic films, dimensional change of the plastic films with heat has often resulted in production of off-specification products, leading to faulty electrical connections between the flexible printed circuit and other electronic components such as connectors.